AudacityException is an abstract base class for exceptions generated by
Audacity.
GuardedCall wraps any function (usually a lambda) in an appropriate catch
block.
It can also accept a second function that defines a catch block action, which
can rethrow or return a value for the GuardedCall.
It can also accept a third function, that defines another, delayed action that
executes in the main thread at idle time if the second function intercepts an
AudacityException and completes without rethrow.
Defaults for the second function simply return void or false. Default for the
third function invokes a virtual method of AudacityException, which for
subclass MessageBoxException, displays a message box.
